1. Field of the Invention
The present invention relates to a disk clamping mechanism for a disk driving device, for clamping a disk-shaped recording medium and, more particularly, to a disk clamping mechanism suitable for clamping a flexible disk, namely, a so-called floppy disk.
2. Description of the Prior Art
A so-called disk, driving device which drives a flexible disk (hereinafter "disk") rotatively for recording information or reproducing the recorded information is constituted so as to center a disk in reference to the central hole of the disk, to clamp the disk along the periphery of the central hole and to drive the disk rotatively. Since such a disk is flexible, various problems arise in clamping the disk. A conventional disk clamping mechanism and problems involved therein will be described in connection with the accompanying drawings.
Referring to FIG. 4 illustrating a conventional disk clamping mechanism, a spindle hub 2 is attached to the upper end of a spindle 1 which is driven by a motor, not shown, and a flexible hub 4 consisting of a plurality of segments is supported loosely on a shaft 3 so as to be fitted in a cavity 2b defined by the annular wall 2a of the spindle hub 2. A collet 5 is mounted rotatably through a bearing 7 on the shaft 3 above the flexible hub 4. A pressure plate 9 and a carrier 12 are fitted loosely on the shaft 3 above the collet 5. Compression coil springs 6 and 8 are disposed between the hub 4 and the collet 5 and between the bearing 7 and the pressure plate 9, respectively. The compression coil spring 6 functions to separate the hub 4 and the collet 5 from each other, while the compression coil spring 8 functions to apply pressure to a disk 11. The resilient force of the compression coil spring 8 is greater than that of the compression coil spring 6. An E-ring 10 is attached to the upper end of the shaft 3 to retain the hub 4, the collet 5, the pressure plate 9 and other members mounted on the shaft 3.
In clamping the disk 11 by the disk clamping mechanism thus constituted, first the disk 11 is mounted on the spindle hub 2 at a predetermined position, and then the carrier 12 is lowered to move the members mounted on the shaft 3 toward the spindle hub 2. Then, the lower portion of the hub 4 enters the cavity 2b enclosed by the annular wall 2a of the spindle hub 2, and then the lower surface of the flange 4a of the hub 4 is brought into contact through the disk 11 with the spindle hub 2. As the carrier 12 is lowered further, the collet 5 comes into contact with the inner taper surface 4b of the hub 4, and thereby the further downward movement of the collet 5 relative to the hub 4 is limited. As the carrier 12 is lowered further thereafter, the collet 5 is forcibly pushed into the central portion of the hub 4 by the resilient force of the compression coil spring 8 to expand the segments of the hub 4 so that the disk 11 placed on the upper surface of the annular wall 2a of the spindle hub is centered and is clamped between the spindle hub 2 and the hub 4. Then, the spindle hub 2 is rotated by the motor so that the spindle hub 2, the hub 4, the collet 5 and the disk 11 rotate together for recording or reproducing. The smooth rotation of the rotary members of the disk clamping mechanism and the disk 11 is secured by the bearing 7. However, such a configuration has a problem that mounting the rotary members, namely, the hub 4, the collet 5 and the bearing 7, on the shaft 3 requires the axial length of the shaft 3 to be large and the construction of the bearing 7 to be complicated. An invention disclosed in Unexamined Japanese Patent Publication (Kokai) No. 59-22272 was proposed to solve such a problem. As illustrated in FIG. 5, this proposed invention employs a rotary member and a nonrotational member interconnected so as to be able to clamp the disk, instead of employing a shaft member, such as the shaft 3, for supporting the rotary members. This proposed invention will be described hereinafter with reference to FIG. 5.
The disk clamping mechanism comprises a spindle hub 2, a flexible hub 4 capable of being fitted in a cavity 2b enclosed by the annular wall 2a of the spindle hub 2 and pressing a disk 11 with the flange 4a thereof, a collet 5 capable of pushing the lower inside of the hub 4, a ball retainer 17 engaging the collet 5 and having a central ball retaining protrusion 17a, a ball 13 retained in the central ball retaining protrusion 17a, a spring seat 15 rotatably placed on the ball 13, a holder 16 for holding a spring 14 and the spring seat 15, a carrier 12 capable of moving the collet 5 and the hub 4 downward by the holder 16, and a spring 18 interposed between the hub 4 and the collet 5 to separate the hub 4 and the collet 5 from each other. The hub 4, the collet 5, the ball retainer 17 and the spring 18 are rotary members, while the carrier 12, the holder 16, the spring 14 and the spring seat 15 are nonrotational members. Only the single ball 13 is interposed between the rotary members and the nonrotational members and functions as both a radial bearing and a thrust bearing.
This disk clamping mechanism does not need the shaft 3 and the bearing 7 of the above-mentioned disk clamping mechanism.
Nevertheless, both the above-mentioned conventional disk clamping mechanisms have a problem in centering the disk 11 on the spindle hub 2 and clamping the same between the spindle hub 2 and the flexible hub 4. When the hub 4 is brought into contact with the annular wall 2a of the spindle hub 2 in a tilted position, the tilt of the hub 4 is enhanced by the annular wall 2a, and hence the centering of the disk 11 cannot be achieved satisfactorily. Since the hub 4 is pushed further toward the spindle 1 in a tilted position by the collet 5, the accurate centering of the disk 11 is impossible. Furthermore, since the hub 4 is pressed against the disk 11 in a tilted position, the uniform and accurate clamping of the disk is impossible. In the worst case, part of the periphery of the central hole of the disk 11 is bent accidentally toward the annular wall 2a of the spindle hub 2.
Such a problem is considered to be due to a reason that the center 0 of the swing motion of the hub 4 and the collet 5 is located above a contact edge 4c where the hub 4 comes into contact first with the annular wall 2a of the spindle hub 2. When the center 0 of swing motion is located above the contact edge 4c, a reaction force A acting perpendiculary to the inner surface of the annular wall 2a and a reaction force acting in parallel to the inner surface of the annular wall 2 act on the hub 4 when the hub 4 is brought into contact with the annular wall 2 of the spindle hub 2 in a tilted position as illustrated in FIG. 6. The reaction force A perpendicular to the inner surface of the annular wall 2a produces a moment M.sub.1 of force tending to turn the hub 4 and the collet 5 in a clockwise direction, as viewed in FIG. 6. As obvious from FIG. 6, the moment M.sub.1 of force tends to increase the tilt of the hub 4, and hence the hub 4 is unable to center the disk accurately when tilted in such a position. On the other hand, when the center 0 of swing motion of the hub 4 and the collet 5 is located below the disk clamping surface as illustrated in FIG. 7, a moment M.sub.2 of force tending to turn the hub 4 in a counterclockwise direction acts on the hub 4, as viewed in FIG. 7. This moment M.sub.2 of force tends to bring the hub 4 into alignment with the inner surface of the annular wall 2a, and hence the hub 4 is brought naturally into alignment with the annular wall 2a of the spindle hub 2 as the hub 4 and the collet 5 are moved deeper into the cavity 2b of the spindle hub 2.